Casting defects such as pinholes, shrinkage cavity (porosity) or the like due to solidification and shrinkage of the molten metal in the process of casting metals such as aluminum alloy, magnesium alloy, titanium alloy or the like air bubbles or hydrogen gas bubbles in dendrite trees are generated in the solidification process and grow with the solidification progress of the molten metal.
The hydrogen gas bubbles which form a nucleus of casting defects are generated when ambient pressure in a pressure container acting on the molten metal is lower than the hydrogen gas partial pressure in the molten metal, and the hydrogen gas partial pressure is sharply increased as the liquid phase ratio lowers.
To prevent the hydrogen gas bubbles from forming, it is effective to apply ambient pressure higher than the hydrogen gas partial pressure onto the molten metal at the stage prior to the solidification of the molten metal. A casting method disposing a casting mold and a furnace in airtight pressure containers and applying higher pressure than the atmospheric pressure to the pressure containers was invented in Bulgaria in the 1960s, which is widely known as Counter Pressure Casting.
In this counter pressure casting, as shown in FIG. 18 describing pressure control pattern, the holding furnace and the casting mold in the pressure containers are applied the same pressure from the atmospheric pressure to the set pressure P1, then, the pressure in the casting mold side is lowered while keeping that in the holding furnace side at the set level by which the molten metal starts to be charged into the casting mold. Then, after the charging of the molten metal is completed at T2, the pressures in both sides are maintained at certain levels from T2 to T3. After T3, the casting mold side pressure is increased to the holding furnace side pressure to dissolve the differential pressure. thereby the molten metal is returned to the holding furnace at T4. Further, after T4, the process discharging the gas from the pressure containers to the atmosphere starts to complete the casting of one cycle at T5.
As to the above Counter Pressure Casting, Japanese Patent Laid-open No. 186259/1989 and Japanese Patent Laid-open No. 278949/1989 disclosed casting methods characterized by providing the differential pressure between the casting mold side and the holding furnace side at from 0.5 to 30% of the maximum pressure; a method applying and holding the pressure of from 3 to 7 kgf/cm.sup.2 to the pressure containers, then adjusting the differential pressure at from 3 to 30% of the holding pressure; and a method increasing and holding the pressure of the containers at from 7 to 30 kgf/cm.sup.2, then adjusting the differential pressure at from 0.5 to 10% of the holding pressure. Further, Japanese Patent Laid-open No. 187247/1990 disclosed a casting method characterized by a following pressure controlling: applying a given pressure to both containers, retention thereof, generation of the differential pressure between both containers and its retention, and decrease of pressure to the atmospheric pressure.
But, the above conventional counter pressure casting has the following problems.
As shown in FIG. 18, the pressures of the furnace side and casting mold side have to be previously increased go P1 which is the maximum pressure in the process before T1 when the molten metal starts to be charged into the casting mold. This makes the duration till T1 long resulting in its low productivity in industrial application.
In order to decrease the period till T1 to improve the productivity, an air current has to be blown into the both containers at a high speed, by which the molten metal within the furnace side container is stirred causing the generation of oxides of the molten metal. Such oxides are mixed with the casting as non-metal inclusions providing the deterioration of the casting. Such non-metal inclusions cause internal or external defects and poor strength in the casting.
Further, the conventional counter pressure casting mentioned above is a method to charge the molten metal into the casting mold by either of the casting mold side pressure reduction method or the furnace side pressure increase method, and the differential pressure is to increase while forming a simple primary curve, Since the differential pressure increasing speed is kept statically even after the completion of charging of the molten metal, unequal solidification proceeds, and feeding head effect from the holding furnace side cannot be expected. As a result, casting defects are left behind to cause internal or external defects and poor strength in the product.
Such defects become particularly obvious when a thin-wall part with a complicated shape, thick-wall part or a material that is difficult to cast is cast, and can not be removed completely.
In addition, in the conventional counter pressure casting shown in FIG. 18, the compressed pressure in both containers is entirely discharged into the atmosphere through an exhaust pipe after solidifying so that the molten metal is returned to the holding furnace. Therefore. The molten metal moves up and down between x in FIG. 19 showing a conventional counter pressure casting device for every casting cycle, wherein x represents the distance between the molten metal surface in the furnace and the molten metal highest position in the casting mold. As the above casting work progresses, the surface of the molten metal in the furnace gradually lowers resulting in appearance of y in FIG. 19 representing the difference between the initial and final height of the molten metal in the furnace. This causes the molten metal temperature, and the necessary charging pressure and time to be changed, and accordingly, affects the quality of the casting as dispersing.
Furthermore, as a result of the molten metal moving up and down within molten metal feeding pipe, turbulent flow is induced in the molten metal in the furnace causing gas entrainment and other defects.